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Date: Fri, 2 Apr 93 14:53:28 CST
From: gav@astro.as.utexas.edu (Gerard de Vaucouleurs)
Subject: SN1993J memo
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Notes on M 81 and SN1993J\\
G. de Vaucouleurs\\
{\it Department of Astronomy, University of Texas, Austin, TX 78712}\\
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1. The mean extinction-corrected distance modulus of M 81 is
(m $-$ M)$_{\circ}$ = 27.75 $\pm$ 0.1 (mean and median of 11 estimates
by seven different authors or teams using ten different methods, 1978-1991).
Extinction corrections are uncertain, however, with A$_{B}$ varying from
0.1 to 0.4. \\ \par
2. The V-band magnitudes (ranging from 8.9 to 15.3) of two dozen comparison
stars in the M 81 field are given in the Thompson \& Bryan 'Atlas of SN
Search Charts' (Cambridge U. Press, 1988) which observers are urged to use.
In particular stars A and C mentioned in the IAU Circulars are shown at
V(A) = 11.9 and V(C) = 14.0. \\ \par
3. If V $\simeq$ 10.3 observed on March 30-31 is the maximum, SN1993J is
$\sim$ 0.8 mag brighter than the Type II SN 1970 in M101 (neglecting color
and extinction corrections). Assume that it was a good match to SN1993J, then,
if the distance modulus of M 101 is
(m $-$ M)$_{o}$ = 28.6 $\pm$ 0.1 (mean and median of 20 estimates by a dozen
sources, 1973-1986), the distance modulus of SN1993J and M 81 is
(m $-$ M) $\simeq$ 28.6 - 0.8 = 27.8 (consistent with mean above) and
the absolute magnitude of SN1993J at maximum is M$_{V} \simeq - 17.5$,
about one magnitude brighter than average for type II SNae on the short
distance scale (de Vaucouleurs, ApJ, 227, 753, 1979). \\ \par
4. If the post-maximum light curve of SN1993J follows the average for
`plateau-type' Type II SNae, the plateau phase will begin 40-50 days past
maximum and at a level $\simeq$ 2.0 mag below maximum (de Vaucouleurs, in
`SNae and Supernova Remnants', Reidel, 1974, p. 203), or about m $\sim$ 12.3
(again neglecting as yet unknown color and extinction corrections). \\ \par
5. The plateau phase should end $\sim$ 100 days after the initial
outburst or $\sim$ 90 days past maximum, at the end of June 1993.
It will be important to closely monitor (photometry, spectroscopy) the
onset of the rapid decline phase following to see if the break is as sudden
and sharp as in, e.g. SN1923 in M83, or is a more progressive transition.
The differences between SNIIae at the end of the plateau phase are not yet
understood.\\ \par
6. For comparisons with SN1987A (light echo, etc) remember that the distance
of M 81 is about 75 times that of the LMC (1" = 17 pc).
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